The present invention relates to color reproduction in a digital color image conversion system.
There is a strong demand for high quality in digital color image from high to low ends due to tremendous price down of computers and peripheral equipment.
In a color printing system, color image data composed of three-dimensional color signals supplied to a personal computer from a color image scanner is displayed on a color monitor and also printed by a color printer.
It is required that the original color image be reproduced in high quality on both a displayed color image and a printed color image. However, since the monitor and the printer have different color reproduction characteristics, the displayed and printed color images would be produced with different color if the color image supplied to the monitor were supplied to the printer with no particular color processing.
International Color Consortium (ICC) profile has been adopted to avoid this problem. The profile has files in which color reproduction characteristics for each device such as a color monitor and a color printer. Using the profile, a personal computer accepts color reproduction characteristics for the devices to reproduce the same color image on the monitor and printer, but still incomplete.
In a system with the ICC profile, an image data as RGB color signal data composed of red, green and blue is supplied to a personal computer. The personal computer takes addresses used for converting the RGB color signal data, with a color conversion table, into CMY color signal data composed of cyan, magenta and yellow to be printed on a color printer. The upper four bits of eight bits of the address.
The RGB color signal data are converted into main data of the CMY color signal data with the addresses on the color conversion table.
Interpolation coefficients are next obtained to generate interpolation data by an interpolation calculation such as eight-point interpolation using the interpolation coefficients and the lower four bits of the RGB color signal data. The interpolation data and the main data are synthesized to generate image data as the CMY color signal data. Here, the eight-point interpolation is to obtain a color on color space by interpolation using colors on eight points around a point on which the color to be obtained is located.
The data on the color conversion table and the interpolation coefficients are stored in a memory of the printer.
Through the color conversion explained above, the color reproductivity of the color monitor supplied the RGB color signal data and that of the color printer supplied the CMY color signal data are optimized.
In another method with the ICC profile, a personal computer first accepts image data A as color signal data corresponding to colors included in a first coloring system. Addresses on a color conversion table are taken under the same way as described above. Color conversion data are then taken using the address on the color conversion table.
Color conversion data interpolation is conducted to generate interpolation data. The interpolation data and the color conversion data are synthesized to generate image data A' corresponding to colors included in a second coloring system.
Other addresses on the color conversion table are taken under the same way as described above. Color conversion data are taken using the addresses on the color conversion table.
Color conversion data interpolation is conducted again to generate interpolation data. The interpolation data and the color conversion data are synthesized to generate image data B corresponding to colors included in a third coloring system.
Through the other color conversion explained above, the color reproductivity of the color monitor supplied the RGB color signal data and that of the color printer supplied the CMY color signal data are optimized.
The two methods define the color reproduction characteristics per device with inflexibility to reproduce the same color on different devices. However, evaluation of reproduced color depends on viewers and observing environment, etc.
Further, modification of color reproduction characteristics of a color printer can only be done by changing color modification parameters and printing out. However, printing out takes time and it costs too much. Further, the color reproduction characteristics of a color monitor and a color printer are different from each other. The color image printed by the printer thus cannot be reproduced on the monitor with identical colors.
Further, it is not so rare for an operator not to be satisfied with a color image reproduced on the monitor. Not only this, a color conversion table on a color printer includes sampled data ranging 8 to 16 bits, for example. Modification of a portion of the color conversion table thus generates distortion on the modified portion. This results in unnatural color reproduction.